Transmission system



Patented July 21, 1931 rares raies GEORGE caisson, or naar oni/irren, new arrasar, .assreivoa To enmarcan TELE- rrrcrra ann @trennen cera-rarer, n scares-Arien or New YORK i TRANSMISSION SYSTEM Application led November 20, 1929. Serial No. 408,580.

rllhis invention relates to telephone transmission systems and more particularly to transmission systems involving a long secticn of submarine cable.

In order that wire telephone transmission may take place between points separated by large body 'of water, as, for example, is the case in telephoning between New York and London, it is necessary that at least one long section of submarine `cable be incorpcrated in the transmission circuit. Because of the expense of constructing such a cable, it is impractical to provide two cables for fourwire operation, and therefore the one long section of submarine cable, for example, be tween Newfoundland and lreland, must be used for two-way transmission. lt is however, quite feasible to operate on a four-wire basis those portions of the circuit which eX- tend between the ends of the cable and the terminal switching stations at say New York and London. Due to various causes substantial echoes may occur in the four-wire part of the circuit.

lith reference to the long section of submarine cable, which is operated on a twoway basis, the attenuation is enormous and although such cable is relatively free from external disturbing influences it will at least have a noise level determined by the resistance noise (or noise due to, thermal agitation) of the cable. ln order, therefore, that the currents received after transmission over the cable have sui'licient volume to be above this noise level, the voice energy must be applied at the sending end of the cable at a very high volume level, so that the difference in level between the sending venergy and receiving energy at a given end-of the cable is very much greater than is the ease with any type of telephone system heretofore enipleyed.

lith this great disparity between applied and received energy at the ends of a cable, it is obviously necessary that voice operated switching arrangements be provided for preventing singing in the four-wire circuit, as well as for suppressing echoes. Furthermore, it is necessary to locate the voice operated arrangement adjacent the ends of the cable. Thisis for the reason that the recei fing amplier must be very sensitive in order to step the weak attenuated currents incoming from the cable up to a normal volume for transmission over the receivingline of the four-wire circuit, and it is therefore necessary to protect such sensitive receiving apparatus from the relatively enormous echo currents coming in from the cable in response to transmission going into the cable from the other side of the four-wire circuit. lt is also necessary to protect the receiver from the enormous transmitting currents which might enter the receiving circuit from the ransmitting path of the four-wire circuit. n

ln the operation of echo suppressor equipment in connection with the long submarine able, a factor which must be considered is the so-called relaxing current. rlhe characteristic impedance of the cable will involve a considerable reactance component. lVhen currents of large energy are applied to one end of the circuit for transmission to the other end, energy is stored up in the reactance of the cable, and, when the voice potential ceases a unidirectional gradually decaying current flows out of the cable because of the energy stored up in its reactance. Since the reactance is large, and the yapplied energy must necessarily be large (because of the great attentuation of the cable), this relaxing current will, at the instant when the applied energyceases, be large and may require a considerable time before it decays to a value small enough to 'cause no disturbance in the sensitive receiving and voice-operated appa.- ratus associated with the cable at its terminal. rlhis relaxing current may therefore be for a considerable period of time very much greater in magnitude than the signaling currents which are to be received over the cable, and, if nothing is done to reduce its effect, the voice-operated relays for disabling the receiving path of the four-wire circuit at the cable terminal might have to be pfroyi'ded with a hang-over much longer than is necessary to take care of the echoes returned from the cable. l

.lt is therefore proposed., accordance with the present invention, to reduce the elect of the relaxing current. This may be accomplished by associating with the terminal of the cable a network having such impedance characteristic that the impedance of the cable as seen through the network will be a substantially pure resistance over the range of frequencies which the cable is to transmit. `With such an arrangement the reactance of the combination comprising the cable and the impedance equalizing network, will approach zero to a degree dependent upon the degree of compensation, so that in the ideal case no relaxing current will flow out of the terminals of the network to the receiving side of the four-wire circuit. In any event in practice the initial value of the relaxing current will be very much decreased so that this factor will be within the limits of the echo effects so far as the hang-over time of the voiceoperated relays is concerned.

In the practice of the Vpresent invention use may be made of any of several known types of networks which, when associated with a smooth loaded line, will render the characteristic impedance of the combination substantially a pure resistance, it being the purpose of the present invention to utilize the fact that giving to the circuit a characteristic impedance of this nature will prevent, or at least diminish, the relaxing currents of the cable. The invention also includes the use of such terminal connections between the cable and the four-wire circuit, and the employment of such types of voiceoperated devices, as will be most effective in the operation of a system provided with impedance equalizers utilized for the purposes above stated.

Other features of the invention will be clear from the following detailed description thereof when read in connection with the accompanying drawings, in which Figure l is a schematic diagramof the apparatus associated with the four-wire circuit at one end of a cable, it being assumed that similar apparatus will be provided at the opposite end of the cable, and Figs. 2 to l()l inclusive, are various forms of impedance equalizer-s which may be used in connection with the cable for reducing relaxing currents.

In Fig. l, only the apparatus associated with the four-wire circuit at the cable terminal is shown, and the apparatus at the other end o-f the four-wire circuit may be of any known type for enabling a subscriber to talk over the transmitting side of the fourwire circuit and to receive telephone messages over the receiving side of the circuit. At the cable terminal the cable X is connected through a transformer TT to a transmitting line TL of a four-wire circuit, the corresponding receiving line RL of the four-wire circuit being associated with the cable through a transformer RT. It is not practical to connect the paths TL and RL in multiple to the cable X, as the voice-operated contacts for disabling the transmitting path would have the effect of short-circuiting the receiving path and vice versa.. Accordingly, the secondary windings of the transformer TT, and the primary winding of the transformer RT are connected in series. Owing to the enormous difference in transmission level between the sending energy from the transmitting circuit TL and the receiving energy transferred from the cable to the receiving line RL, shielding is provided to protect the low level parts of the receiving circuit from the high level transmitting apparatus. This is indicated by the dotted line rectangles in the figures which surround certain portions of the receiving and transmitting equipment respectively.

The transmitting line includes a line amplifier `W2 for stepping the transmitted energy up to any desired volume. It also includes an auxiliary volume regulator VR2 to bring the volume to some preassigned value, for example 5 decibels above reference volume and a transmitting amplifier D2 for bringing the linal volume to be applied to the cable up to the desired level which, as indicated, may be +30 db. Normally, the transmitting line is disabled ust ahead of the transformer TT by means of the contacts of the relay E2 which is controlled by an amplifier-detector arrangement F2 bridged across the transmitting line ahead of a delay circuit O2 which may be provided for the purpose of enabling the relay E2 to operate before the voice currents arrive at the point where the circuitis disabled. It will be noted that the contacts of the relay E2 are bridged across the output circuit of the amplitier D2 rather than across the input circuit, so that any noise arising within the amplilier will be prevented from reaching the cable and the receiving side of the system. l/Vhile this contact has the effect of short circuiting the secondary windings of the transformer TT, the fact that the primary windings of the transformer RT is in series with the secondary windings of the transformer TT, prevents the short circuit from being ecctive in the receiving path RL.

The amplifier-detector F2 also controls a relay H2 by means of which the receiving line RL may be disabled. Normally, the receiving circuit is operatively connected over the back Contact 3 of the relay H2 to the input of a low power amplifier P2 which in turn is connected to a high power amplifier P2 for bringing the weak received currents in the cable up to the desired value for transmission over the receiving line EL, Assuming that the attenuation of the cable is 150 db. and that the currents are applied at the distant end at a volume of 30 db., the current will be received from the cable at a volume level of l20 db. If it is assumed that the current is to be applied to the receiving line RL at a .volume level Lof -5 db., the Voverall gain of the amplifiers P2 and P2 should be 125 db. These iig-lires are, `of course, merely given for purposes of illustration. Y

An attenuation equalizer K2 may be connected between the two amplifiers P2 and P2 for equalizing the attenua-tion of the currents transmitted .over the cable, the cable, of coure, having the effect lof attenuating the high frequencies to a much greater extent than the low frequencies. An amplifier-detector arrangement B2 is associated with the receiving line and controls a relay A2 for disabling the transmitting line TL. The contact eontrolled by the relay A2 is preferably associated With the line TL ahead of the volume regulator VR2 in order t-l at Where an automatically adjusted volume regulator is lISCd it Will not be adjusted by echo current-s.

The operation Will now be described briefly. Waves originating at a subscribers telephone come in over the line TL and after passing through the line amplifier W2 and the volume regulator VR2, operate the amplifier- .deteCOr F2 to cause the actuation of the relays E2 and H2. The delay circuit O2 enables the relay E2 to .open the short circuit across the input side of the transformer TT before the voice Waves arrive at this point, thus lavoiding initial clipping. This voice-operated apparatus must operate on all voice r Waves, since failure of the relay E2 to operate Would prevent the transmission of speech to the cable, While failure of the relay H2 to operate would permit powerful echoes to pass into the receiving line BL and operate the relay A2, thus interrupting the outgoing speech.

The relay H2 is provided With three contacts, l, 2 and 3, so associated that cont-act 1 makes before contact 2 makes, While contact 2 makes .before contact S'breaks. The result of ymaking contact l is to short circuit the secondary windings of the transformer RT so as to prevent the effect of an open circuit impedance across the terminals of the cable X due to the opening of contact 3. The opening of contact 3 is necessary on the other hand, because the short circuit across the secondary transformer RT Would not be sufficient to cut down the transmission from the transmitting line TL which is applied to the cable at the volume level of -l- 30 db. and hence would arrive in the receiving channel at a volume level of 150 db. above the normal received currents. The contact 2 is necessary because the receiving amplifier combination B2TP2 Vis very sensitive, and its input should be short circuitcd by the contact 2 to prevent it from picking up noise through the capacity of the open contact 3. Relay H2 must operate before the contact of relay E2 opens, otherwise the high power noise from the transmitting side would cut into the receiver momentarily., thus operating the voice-operated relay A2 associated With the receiving line YRL to disable the transmitting line TL. lVhile it may happen that both the transmit- .ting line and receiving line are short-circuited momentarily due to the contact 1 of the relay H2 being closed before the Contact of relay E2 opens, this does no harm. While it has the effect of momentarily terminating the cable X in short circuit instead of in its .own impedance, thereby causing echo currents to be reiiected back into the cable, such .echo currents will be so greatly attenuated before arrival at the distant end as to be negligible.

lhen Waves come in over the cable X they pass Without hindrance over the contact 3 of the relay H2 (assuming that relay H2 is unoperated) through the receiving amplifier P2, equalizer K2` and high power amplifier P2 and thence over the line RL. In passing, these Waves cause the amplifier-detector B2 to actuate the relay A2, which latter short circuits the transmitting side of the circuit and so prevents operation of the anti-singing relay E2 and the echo suppressor H2 either by voice or echoes during the passage of incoming Waves. The relay A2 is completely protected from land line and terminal noise When speech is not being received over the cable. The line amplifier M2 prevents noise coming baclr from the receiving line RL to operate the relay A2, While at the distant end of the cable the anti-singing relay corresponding to E2 prevents any noise from entering the cable at that terminal when speech is not passing, and consequently the only noise coming from that direction to which the relay A2 is subjected is that arising in the submarine cable itself and in the apparatus such as the amplifier P2, equalizer K2, etc. This noise will be due principally to thermal agitation, and, after passing through the equalizer K2 will be much stronger at high frequencies than at low frequencies. The amplifier-detector B2 should therefore be most sensitive to the important voice frequencies and should be quite insensitive to the higher voice frequencies, thus reducing the liability of the relay A2 being operated by thermal agitation.

Owing to the large transmission loss of the submarine cable, Which may be assumed to be in the neighborhood of 150 db'. at the upper end of t-he voice range (about 3000 cycles) certain precautions must be taken that ordinarily are not necessary. Assuming that current from the transmitting side is applied to the cable at +30 db. and that current applied at the distant end is received from the cable lat ak level of 120 db., the total difference in level is 150 db. Consequently, if the circuit from the output of the amplifier D2 to the` transformer TT is not Vll() perfectly balanced longitudinally, an unbalanced or longitudinal voltage will act through the system and transfer noise to the cable which is great-er in volume than received transmission even when the transmit- -ting circuit is short circuited by the relay E2. This effect will take place regardless of the .loss to normal transmission caused by the contact of the relay E2, the longitudinal current flowing in effect through the capacity between the windings of the transformer TT. This longitudinal effect can be greatly reduced by placing a shield grounded to the cable sheath between the'line and drop windings of the transformer TT. A similar shield, connected to the ciiice ground, should also be provided between the windings of the output transformer (not shown) of the amplifier D2. The effect of these shields is to shunt to ground the displacement current which tends to pass through the capacity of the windings. Also, it is desirable that the secondary winding of the output transformer of the amplifier D2 and the primary winding of the transformer TT be made in two closely balanced and closely coupled parts with grounds at the midpoints. The series resistances of the conductors connecting the amplifier D2 to the transformer TT should also be accurately balanced.

As already described the relay H2 is arranged to disable the receiving circuit by opening the Contact 3. It is undesirable to open both sides of the receiving circuit by contacts of the relay A2, as a momentary unbalance would occur if the two contacts did not open simultaneously, and this might produce enough disturbance to cause false operation of the relay A2, thereby clipping speech outgoing from the transmission line L to the cable X. In order to make possible the use of this unsymmetrical arrangement of the relay H2, the transformer RT is provided. The transformer also serves to insulate the cable from the input circuit of the amplifier P2 and to shield said amplifier from the longitudinal voltages on the conductors. F or this purpose a shield between the windings of the transformer RT is rovided which is connected to the sheath of the cable X.

The operation so far described has not taken into any account the effect of the socalled relaxing current from the cable X. The loaded submarine cable has a characteristic impedance which is not a pure resistance but involves a very considerable reactive component. As has been previously stated, the voice currents are applied to the cable from the line TL at a very high volume level, and the voice potentials thus store up in the reacta-nce of the cable a very large volume of energy, with the result that when the applied voice potential ceases, the energy stored up in the reactance causes a gradually decaying unidirectional current to flow out of the cable. This current, at the moment of cessation of voice waves, is so large in magnitude that it might produce very serious disturbing effects in the receiving circuit. Unless some means were provided to reduce its eifect-the relaxing current from the cable might require a longer hang-over time for the relay H2 than might be required for reflected currents due to irregularities in the cable. To take care of this condition it is proposed to interconnect between the terminals of the cable X and the transformer connections associating the cable with the transmitting line TL and the receiving line RL, a so-called impedance equalizer' indicated schematically at Y in Figure 1. This impedan ce equalizer is a network of known type which is designed to have such an impedance characteristic that the overall impedance of the combination of the cable X and the network Y will appear to be a pure resistance as seen through the equalizer network Y. If the impedance thus seen is an absolutely pure resistance no relaxing current would flow out of the cable into the receiving line RL as the overall combination would have no reactance as seen from its terminals. Even though this ideal effect can only be approximated in the design of the impedance equalizer Y, so that there will be some reinanent reactance in the characteristic impedance of the combination, the relaxing current may be reduced to such a small initial value that it will have decayed to an ineffective value` within the hang-over time required by the relay H2 to take care of echo currents reflected from the cable.

Various known types of impedance equalizers may be designed for producing the aforementioned results. For example, in Fig. 2, the equalizer Y may take the form of a shunt across the line comprising a series combination of resistance R and inductance L. Such an equalizer will produce a moderate reduction in the overall reactance component of the combination of equalizer and cable. This type of equalizer structure and its method of design are fully disclosed in the application of Ray S. Hoyt, Serial No. 408,577 filed November 20, 1929.

Vhere greater precision of equalization is desired, shunt equalizersl of any of the types illustrated in Figs. 3 to 6 inclusive, may be employed. These four types of equalizers are all equivalent electrically, and their structures are claimed and their methods of Vdesign are disclosed in the application of Ray S. Hoyt, Serial No. 408,578 tiled November 20, 1929. The equalizer of Fig. 3 comprises a shunt arrangement including a resistance R1 in series with an inductance L2, these elements being in turn conected in series with a parallel combination of an inductance L3 and a resistance G3. In the equalizer of Fig. 4t, the inductances L2 and L3, andthe resistance G3, are replacedby an inductance L., in parallel with a series combi-k nation of inductance L5 andl resistance G5.

In Fig". 5, the shunt element comprises an in-v ductance L6 in series with a parallel combination of resistance Gg shu'nted by a` series series type of equalizer. Four forms which' the series type of equalizer may 'assume are illustrated in Figs. 7, 8', 9 and l0. These four forms of equalizer are claimed and' their methods of designl are disclosed in the ap'- p'lication of Ray S. Hoyt, Serial No. 408,579 il'ed November' 20; 1929.

The series type of'equalizer has the ob' jection that inorder to provide longitudinal balance between theV two sides of the cable system the equalizer' must' be split in half, and on'e'lialf be included in each line conductor, thusrequiring' twice asl many inductances and resistanc'esv asin' the cese of the shunty type impedance equalizer. In the case of Figure 7, t'he series equalizer connected in one line'wire comprises a resistance'S shuntl ed by a.` combination' consisting of air induc tance L2v in series with a parallelA combination of' inductance L3 and resistance G3. It

willbe understood, ofY course', that similar elements are serially connected in theother line' conductor. VIn the case'of'Fig. 8, the re-l sistance; S 1s shuntedby 4a combination of inductance L4- arrangedin parallel with' aV series combination of'resistanc'e yGr, and' inductanceLm In the case ofFig.I 9,the`equalizer'c'onnected in oneline-wire comprises an inducta'nce LG shunted by a com'bination'consist'ing of a resistance @r6V in serieswith' a parallelicombination otresistance G2 and in'- di'ic'tance L7. VIn the case of Fig.4 10, the equ'al4 g izerino'ne line 'wirec'ompri'ses two units, each comprisingan'inductance shunt'edby a re-z sista'nce.v For example, the `inductance LS; shunt'ed by resistance G'Sformsone u'nit, andi the'in'duct'ance L ,shunted by theresistance' G9 forms a second uni-t, the'vtwo units being1 in' ser-ies with each other.

The present invention, from tlie standpoint of' reduction of relaxing fcurren'ts, utilizes the principle thatwhen a known typepof impedeA ance equalizer is combined witha loadedsubf marine cable' sothat the impedance, looking is because thel energy stored'up in thereL actance of the cable and tending to cause a relaxing current to llow from the cable', is balanced by opposing energy stored up in ther opposing reactance of. the network. The specitic types of equalizer structures per se and their methods or design are not a` part or the present invention, and hence are not disclosed in detail; or claimed herein, but, as al; ready's'tated, form the subject matter of cer tain applications of Ray S. Hoyt previously identified.

It will be obviousthat the general princi-A ples herein disclosed may be embodied in' many other organizations widely diiferent" from those illustrated, without departing from the spirit of the invention as delin'ed in the following claims.

W'hat is" claimed is: n

' l. Ina signaling system, a transmission path of large attenuation and whose characteristicimpedance includesv reactance of such' value that in responseto the cessation of afp"-V plied signaling currents a relaxing current will flow out or' the path having' van initial value which is large' as compared with at-` tenuated signaling currents received over' the`V path, and means to substantially suppress" said relaxing current compi, a network associated with the Vtransmisslii path, said network having such impedance clia'racter'i's-Av ti'c that the impedance or' the combination of network and transmission path will be sub'- st'antially a pure resistance.

2." In', a1 signaling' system, a' transmission' path of large attenuation and whose charac,- teristic impedance includesy reactance of sfuc'hf values that in response to the cessation of applied signaling currents a relaxing current will llow out ofthe path having an initial' value which is large as compared' with at tenuated signaling currents received over the path, and means tov substantially suppress saidA relaxing' current comprising a network associated with the transmission path, saidl networkhavingV such impedance ch'aracteris-` tic that theimp'edan'c'e of the combination 'of' network and transmission path willbe'substantially free of reactanc'e in which energy to `produce relaxing currents may be built up.

3. In a signalingsysten'i, a transmission path of large'attenuation and whose characf' teristic impedance includes reactance of such value that in response to the cessation of applied signaling currents a relaxii'ig' current will llow out of the path having an initialA value which isla'rge as compared witli'attenuated,` signaling currents received over the path, means to substantially suppress said relaxing current comprising a network as'- sociatedwith the transmission path, said net'- wlork Having such impedance"characteristic thatA the impedanc'eof the combination of network' andtransmission path will be substantially a pure resistance, a transmitting line includingmeans to apply signaling' curisc rents of large amplitude to said transmission path, a receiving line including sensitive receiving apparatus to receive attenuated signaling currents from said transmission path, signal operated switching means associated with each of said lines for controlling the other line, and means for so associating said lines with said transmission path that said signal operated switching means will not disable said transmission path.

4. In a signaling system, a transmission path of large attenuation and whose characteristic impedance includes reactance of such value that in response to the cessation of applied signaling currents a relaxing current will flow out of the path having an initial value which is large as compared with attenuated signaling currents received over the path, means to substantially suppress said relaxing current comprising a network associated with the transmission path, said network having such impedance characteristic that the impedance of the combination of network and transmission path will be substantially free of reactance in which energy to produce relaxing currents may be built up, a. transmitting line including means to apply signaling currents of large amplitude to said transmission path, a receiving line including sensitive receiving apparatus to receive attenuated signaling currents from said transmission path, signal operated switching means associated with each of said lines for controlling the other line, and means for so associating said lines with said transmission path that said signal operated switching means will not disable said transmission path.

5. In a signaling system, a submarine cable for two-way signal transmission, said cable having large attenuation and its impedance including reactance of such value that in response to the cessation of applied signaling currents a gradually decaying relaxing current will fiow out of the cable with an initial value so great that at the end of a period during which echo currents are reflected from the cable with disturbing effect the decayed relaxing current will be of greater effect than reflected currents, and means to substantially suppress said relaxing current comprising a network associated with the cable, said network having such impedance characteristic that the impedance of the cable as seen through the network will be substantially a pure resistance.

6. In a signaling system, a submarine cable for two-way signal transmission, said cable having large attenuation and its impedance including reactance of such value that in response to the cessation of applied signaling currents a gradually decaying relaxing current will flow out of the cable with an initial value so great that at the end of a period during which echo currents are refiected from L the cable with disturbing effect the decayed relaxing current will be of greater effect than reflected currents, and means to substantially suppress said relaxing current' comprising anetwork associated with the cable, said network having such impedance characteristic that the impedance of the cable as seen through the network will be without suicient reactance in which to build up a substantial relaxing current.

7 In a signaling system, a submarine cable for two-way signal transmission, said cable having large attenuation and its impedance including reactance of such value that in response to the cessation of applied signaling currents a gradually decaying relaxing current will flow out of the cable with an initial value so great that at the end of a period during which echo currents are reflected from the cable with disturbing effect the decayed relaxing current will be of greater effect than reflected currents, means to substantially suppress said relaxing current comprising a network associated with the cable, said network having such impedance characteristic that the impedance of the cable as seen through the network will be substantially a pure resistance, a transmitting line including means to apply signaling currents to the cable with such amplitude that they will arrive at the other end without being attenuated to the level of the resistance noise of the cable, a receiving line including amplifying apparatus sufficiently sensitive to amplify signaling currents so attenuated to a normal transmission level, signal operated switching means associated with each of said lines for controlling the other line, and means for so associating said lines with the same end of said cable that the signal operated switching means when operated to disable either line will not interfere with transmission between the other line and the cable.

8. In a signaling system, a submarine cable for two-way signal transmission, said cable having large attenuation and its impedance including reactance of such value that in response to the cessation of applied signaling currents a gradually decaying relaxing current will flow out of the cable with an initial value so great that at the end of a period during which echo currents are reflected from the cable with disturbing effect the Vdecayed relaxing current will be of greater effect than reflected currents, means to substantially suppress said relaxing current comprising a network associated with the cable, said network having such impedance characteristic that the impedance of the cable as seen through the network will be without suicient reactance in which to build up a substantial relaxing current, a transmitting line including means to apply signaling currents to the cable withv such amplitude that they will arrive at the other end without being attenuated to the level of the resistance noise of the cable,

a receiving line including amplifying apparatus sufficiently sensitive to amplify signaling currents so attenuated to a normal transmission level, a signal operated switching means associated With eaeh of said lines for controlling the other line, and means for so associating said lines with the same end of said cable that the signal operated switching means when operated to disable either line will not interfere With transmission between the other line and the cable.

In testimony whereof, I, have signed m name to this specification this 14th day of November, 1929.

l5 GEORGE CRISSON.

Cil 

